Tetyana Vasylyeva, D.Phil., assistant professor of population health and disease prevention at UC Irvine, and senior author on the study, analyzed the spread of HIV through the virus genetic data. Combining molecular epidemiology with the questionnaire answers, the scientists found that people were crossing the border in both directions and being exposed to HIV despite government efforts to keep the border closed. The scientists then deepened their analysis by applying advanced molecular techniques.

For every patient who tested HIV positive, Vasylyeva and her colleagues isolated the virus, sequenced its RNA and analyzed the genetic diversity in the border region. They investigated how closely the viruses were related genetically—a field of study called phylogenetics, in which scientists construct trees representing how viruses relate to each other.  

After constructing viral phylogenetic trees, the scientists subjected them to a molecular clock analysis. Molecular clock is based on a theory that assumes organisms evolve with a relatively constant rate over time. In this case, applying the molecular clock to the phylogenetic trees enabled the scientists to estimate the timing of cross-border HIV transmission. That further enabled them to identify transmission clusters. “If two or three people have viruses that are very similar to each other, we can assume that the transmission event happened more recently, because there is not enough evolution between these different viruses from these different people,” explained Vasylyeva.

The next step was to combine the survey data and the phylodynamic data. They found that during the 18-month period of their study, nine people contracted HIV, mostly during the pandemic. “Nine sounds like a small number, but it's actually quite a lot of people because in the U.S., HIV incidence is relatively low. We were surprised to see this change in HIV status in such a short amount of time and wanted to look more closely at these clusters,” said Skaathun.

Applying a molecular clock to the clusters, they found a surprise: all of the new clusters had sequences from participants on both sides of the border, indicating that cross-border transmission was happening right when the border was closed. One cluster that grew despite the border closure included two people from San Diego that used drugs in Tijuana. “This shows that efforts to build a higher wall or policies to stop immigration will not mitigate HIV spread,” said Vasylyeva.

Structural risk is a social factor that increases harm. Skaathun says the current study shows that closing the border during the pandemic functioned as a structural risk factor. “The Frontera [border] is one integrated community that is not defined by place of residence. Efforts to end the HIV epidemic in the U.S. also need to be integrated by extending to Tijuana,” she said. 

In 2000, the U.S.-Mexico Border Health Commission was founded to “provide international leadership to improve health and quality of life along the U.S.-Mexico border.” Gudelia Rangel, Ph.D., serves as executive secretary for the commission. Rangel is also a co-author on the current study. “These findings confirm that HIV has no passport,” she says. Like her colleagues, she believes that establishing programs to eliminate HIV transmission at the border, such as harm reduction and substance use treatment services, is a more effective way of preventing viral spread than attempting to close a porous border.

Other authors on this paper include Steffanie A. Strathdee, Carrie L. Nacht, Annick Borquez, Irina Artamonova, Alicia Harvey-Vera, Carlos F. Vera, Caroline Ignacio, Brendon Woodworth, Antoine Chaillon, the University of California San Diego; and Cho-Hee Shrader, Columbia University. 

This research was supported by the National Institutes of Health, the James B. Pendleton Charitable Trust, the San Diego Center for AIDS Research, the National Institute on Drug Abuse, The University of California Office of the President, The James B. Pendleton Charitable Trust, the Center for AIDS Research Translational Virology Core, the National Institute of Allergy and Infectious Diseases, and the Branco Weiss Society in Science Fellowship.

Disclosures: The authors report no conflicts of interest.